1. Field of the Invention
This invention relates to a polishing pad which can be utilized as a polishing pad characterized by being capable of industrially easily fine surface processing and usable as a polishing pad effecting stable planarizing processing, at high polishing rate, materials requiring surface flatness at high level, such as a silicon wafer for semiconductor devices, a memory disk, a magnetic disk, optical materials such as optical lens and reflective mirror, a glass plate, metal etc. The polishing pad of this invention is suitable for use in the step of planarizing particularly a silicon wafer, a device (multi-layer substrate) having an oxide layer, metal layer etc. formed on a silicon wafer, or a silicon wafer before lamination and formation of such layers.
This invention also relates to a method of producing the polishing pad and to a cushion layer for the polishing pad.
2. Description of the Related Art
Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a layer-forming step in order to provide reliable semiconductor connections among various layers used in manufacturing circuits in a producing step of IC, LSI and the like.
Generally, a polishing pad is stuck on a rotatable supporting plate called a platen, while a semiconductor wafer is held on a plate called a polishing head capable of self-rotation. By rotational movement of the two, a relative speed is generated between the platen and the polishing head, and while a solution (slurry) having very fine silica- or ceria-based particles (abrasive grains) suspended in an alkali solution or in an acidic solution is allowed to flow through a gap between the polishing pad and the wafer, to effect polishing and planarizing process. When the polishing pad moves on the surface of the wafer, abrasive grains are pushed at contact points against the surface of the wafer. Accordingly, the surface of the wafer is polished by the sliding dynamic frictional action between the surface of the wafer and the abrasive grains, to reduce the unevenness and surface roughness of the wafer. Such polishing process is usually called CMP (chemical mechanical polishing).
<[I] Polishing Pad>
The known polishing pad for the mirror surface of a semiconductor wafer used in the polishing step include a polishing pad of polyurethane foam type, a polishing pad of polishing cloth type having a polyester nonwoven fabric impregnated with polyurethane resin, and a polishing pad of stacked type having the above 2 pads laminated therein.
As the polishing pad of polyurethane foam type, a polyurethane foam sheet having a void volume of about 30 to 35% is used. Techniques described in Japanese Patent Application National Publication (Laid-Open) No. 8-500622 disclosing a polishing pad comprising fine hollow particles or water-soluble polymer particles dispersed in a matrix resin such as polyurethane are also known.
Among these polishing pads are those formed grooves or holes on the surface of their polishing layer for the purpose of improving the fluidity of slurry and maintaining the slurry. As known techniques of forming surface pattern of a polishing layer in the polishing pad, known techniques of forming surface pattern by a worker with a device such as a cutter, a chisel, or a diamond lathe are disclosed in JP-A 11-48129, JP-A 11-58219 and JP-A 11-70462.
The known polyurethane foam sheet having a void volume of about 30 to 35% as described above is excellent in a local planarization, but exhibits compressibility as low as about 0.5 to 1.0% and is thus poor in cushioning characteristics, to make it difficult to give uniform pressure onto the whole surface of a wafer. Accordingly, polishing processing is carried out usually after the backside of a polyurethane foam sheet is provided separately with a soft cushion layer.
The polishing pad of polyurethane foam type or the polishing pad descried in Japanese Patent Application National Publication (Laid-Open) No. 8-500622 constitutes a polishing layer by itself, and when the polishing surface is worn, the surface is renewed to constitute a polishing layer. That is, the whole of the polishing pad is uniformly elastic and thus has a problem with polishing rate, the uniformity of a material polished, and a difference in step height. That is, there is a problem that when a material constituting a polished surface has a difference in hardness, a softer region is polished in a larger amount, thus failing to achieve flatness at the microscopic level. For polishing, the polishing pad should be provided at the backside (i.e. platen attachment side) with a cushion layer having a polyester nonwoven fabric impregnated with polyurethane resin, thus requiring an additional step of sticking the cushion layer in the method of producing the polishing pad, to make it difficult to cope with demand for reduction in costs.
In these polishing pads, abrasive grains, polished dust etc. are accumulated in voids on the surface of the polishing layer during polishing to reduce the polishing rate, thus periodically necessitating the dressing step of polishing the surface with a head having abrasive grains of diamond deposited thereon, to renew the polishing surface during polishing, but there is a problem that because voids in the polishing pad are not uniformly dispersed and the size and shape of the voids are irregular, the surface renewed by the dressing step is not the same as previous one, to give rise to a difference in polishing characteristics. Further, polishing cannot be conducted during the dressing step to cause a reduction in the efficiency of production. Furthermore, the pad is polished in the dressing step, and thus there is a problem that the pad is consumed in the dressing step in addition to the polishing step.
For fluidizing and maintaining slurry used in polishing, the polishing surface is formed grooves, concentric circles or holes thereon. This processing means include cutting with a chisel, cutting device etc. or pressing with a specified mold, but the cutting means suffer from difficulty in preventing quality variation depending on worker's individual variation, difficulty in changing manufactured patterns, limit to form fine patterns, and generation of burrs to mar the surface of a material polished, while the pressing means has problems such as an increase in costs due to manufacture of a mold and a change, by pressing, in physical properties of a region surrounding the processed region.
As a method of solving the problems in pressing, there is proposed manufacture of a polishing surface by coating a substrate with a liquid photosensitive resin and subsequent photolithography, as described in WO9830356, wherein a photosensitive composition is irradiated with UV rays or laser light to cure irradiated regions in order to remove non-irradiated regions.
When a pad having a certain thickness is manufactured by application of the above liquid photosensitive resin, the liquid resin spreads with time on a substrate, to causes a problem in thickness accuracy. Production of a pad using a spacer etc. to solving this problem causes a reduction in industrial efficiency. Further, the resin is liquid before light exposure, product control (temperature control etc.) is difficult in the process from light exposure to solidification, and the stock of the product is also difficult, to cause a reduction in industrial efficiency. Further, the problem of necessity for the periodical dressing step in the polishing step is still not solved.
An object of this invention is to provide a polishing pad which can be easily subjected to surface processing to form a sheet and grooves, is excellent in thickness accuracy, attains a high polishing rate and achieves a uniform polishing rate.
In a polishing pad of stacked type laminated with a cushion layer, a middle layer is divided into segments to make the elastic characteristics different from those of the polishing layer to improve polishing characteristics, as described in JP-A 11-48131, and in this case too, there are the problems described above. To improve the polishing characteristics of the polishing pad, the polishing layer and other layers are provided with various embossed patterns, but still not solve the above problems.
Another object of this invention is to provide a method of producing a polishing pad which solves the problems described above, is free of quality variation resulting from an individual variation, easily enables a change in processed patterns, enables fine processing, is compatible with various materials to be polished, and is free of burrs upon forming patterns, as well as a method of producing the same.
A still other object of this invention is to provide a polishing pad having a high polishing rate, being excellent in uniformity of a material to be polished and in a difference in step height, and not necessitating stacking a cushion layer on the attachment side for a platen.
The polishing pad of foam type described above is a relatively soft pad of low elastic modulus so that as shown in FIG. 6, the polishing layer 31 itself is deflected so as to follow the shape of a circuit pattern 32 in a semiconductor wafer, and the insulating layer 34 between patterns 33 is polished in excess, to cause a problem with planarizing characteristics at the microscopic level of a material to be polished. In the polishing pad of foam type, there is a limit to an increase in the elastic modulus of the polishing layer, and there is also a limit to improvement in planarizing characteristics.
Some polishing pads with an improvement in elastic modulus out of physical properties of the polishing layer include: (1) a polishing pad having a hydraulic module of 250 psi upon compression of 1 psi when the polishing layer is compressed with 4 to 20 psi (JP-A 6-21028), (2) a polishing pad using a polishing layer having a tensile elastic modulus of 1 MPa to 500 MPa (JP-A 2000-202763), and (3) a polishing pad having an bending elastic modulus of 3500 to 40000 kg/cm2 (JP-A 2001-105300). The polishing pads described in these literatures have improved planarizing characteristics to a certain extent, but it cannot be said that those polishing pads having satisfactory planarizing characteristics are obtained.
A still further object of this invention is to provide a polishing pad excellent in planarizing characteristics of a material to be polished.
A polishing pad using a conventional polyurethane sheet provided with a cushion layer has the following problems.                (1) A nonwoven fabric having continuous pores impregnated with resin is widely used as the cushion layer, but there are problems such as variation among nonwoven fabrics and a change in compression characteristics due to immersion in slurry.        (2) A foamed urethane foam having independent pores comes to be used, but there are still problems such as difficult stabilization of a foamed state in production, significant residual strain resulting from the pores subjected to repeated loading, etc.        
A still other problem of this invention is to provide a cushion layer which can reduce variations in compression characteristics, a change in compression characteristics due to immersion in slurry, and the influence of residual strain of the polishing layer upon repeated loading.
<[II] Slurry-Free Polishing Pad>
For the polishing pad used in CMP, the following techniques are known:
1) A polishing pad having a synthetic leather layer as a polishing layer laminated on an elastic polyurethane layer (U.S. Pat. No. 3,504,457).
2) A polishing pad structured by laminating a foamed polyurethane layer with a nonwoven fabric impregnated with polyurethane (JP-A 6-21028).
3) A polishing pad provided with a polishing surface and a rigid element of selected rigidity and thickness adjacent to the polishing surface and with an elastic element adjacent to the rigid element to endow the rigid element with substantially uniform strength, characterized in that the rigid element and the elastic element give elastic flex strength to the polishing surface to induce the controlled flex of the polishing surface so as to fit it to the whole shape of the surface of the material polished and to maintain rigidity controlled for the local shape of the surface of the material polished (JP-A 06-077185).
4) A polishing cloth comprising a surface layer A having high longitudinal elastic coefficient EA and a lower layer B having low longitudinal elastic coefficient EB, characterized by being provided with a middle layer M having higher longitudinal elastic coefficient than that of the layer B between the layers A and B (JP-A 10-156724).
5) A pad composed of a polishing layer, a middle layer having higher elasticity than that of the polishing layer, and a soft lower layer, wherein the middle layer is divided (JP-A 11-48131).
The polishing pads 1) to 5) described have the following problems:
1) For the uniformity of the whole surface, the elastic polyurethane layer in this system plays a role in making loading applied to a wafer uniform, and since a soft synthetic leather is used as the outermost polishing layer, there is no problem such as scratches, but there is the problem of poor planarizing characteristics in finite regions.
2) In the stacked type of polyurethane and a nonwoven fabric, the nonwoven fabric layer acts the same role as the elastic polyurethane layer in the above-mentioned 1), to achieve uniformity. Further, the polishing layer has a rigid foamed polyurethane layer and is thus superior to the synthetic leather in planarizing characteristics, but does not reach levels required in recent years for improving planarizing characteristics in finite regions and for polishing metal layers. Further, the planarizing characteristics can be improved by further increasing the hardness of the rigid urethane layer, but in this case, scratches occur frequently, thus making this prior art pad unpractical.
3) The structure having a polishing layer, a rigid layer and an elastic layer is constituted so as to have suitable hardness not causing scratches on the polishing layer as the surface layer and to permit the second rigid layer to improve planarizing characteristics deteriorated due to low rigidity. This is to solve the problem in the system in the above-mentioned 2), but in this case, the thickness of the polishing layer is specified to be 0.003 inch or less, and with this thickness given, the polishing layer is also shaved to reduce the longevity of the product.
4) The basic idea in this system is the same as in the above-mentioned 3), and the range of the elastic modulus of each layer is limited to achieve a more efficient range, but in this system, there is no substantial realizing means, thus making production of the polishing pad difficult.
5) The basic idea in this system is also the same as in the above-mentioned 3), but the middle rigid layer is divided in a certain predetermined size to further improve uniformity in the surface of a wafer. However, the step for dividing the layer costs much, thus failing to provide an inexpensive polishing pad.
Further, these polishing pads in 1) to 5) requires expensive slurry to flow during polishing, thus leading to an increase in production costs. Accordingly, a fixed abrasive polishing pad containing abrasive grains in a polishing layer has been developed. Unlike the polishing pad in a free abrasive grain system, the fixed abrasive polishing pad does not require expensive slurry to flow during the polishing step.
As the fixed abrasive polishing pad, for example 6) a polishing pad constituted by mixing cerium oxide particles with foamed urethane resin is disclosed (JP-A 2000-354950, JP-A 2000-354950). In this polishing pad, however, there is a problem that since the density of abrasive grains in the polishing layer is not so high, slurry should be used simultaneously in order to increase the polishing rate.
Further, 7) a polishing pad produced by dispersing abrasive grains in a binder solution in a solvent and coating the dispersion onto a film is disclosed (JP-A 2000-190235). However, there is a problem that this polishing pad comprises the resin and abrasive grains mixed merely in a solvent, thus undergoing aggregation of the grains to generate scratches easily.
Further, 8) a polishing pad produced by secondarily aggregating primary abrasive grains of 0.5 μm or less so as not to contain a binder resin and fixing the resulting granulated particles of 1 to 30 μm via binder resin onto a substrate (JP-A 2000-237962). In this polishing pad, abrasive grains are positively aggregated to introduce the abrasive grains efficiently into the resin, but there is a problem that the aggregates cause scratches easily.
Further, 9) a polishing pad produced by mixing abrasive grains having the maximum particle diameter of 2 μm with a resin material whose particles having an average particle diameter of 50 μm or less are solid at ordinary temperature, then introducing the mixed material into a mold and compression molding it under heating is disclosed (JP-A 2000-190232). However, this polishing pad has a problem that the resin powder is hardly uniformly mixed with the abrasive grains at an initial stage, and when the density of grain particles in the polishing pad is increased, the binder resin is decreased to make molding difficult.
As described above, there is no satisfactory pad in the fixed abrasive polishing pad at present.
A further object of this invention is to provide a polishing pad which is used as a pad for semiconductor wafers in the polishing step of planarizing fine unevenness on a fine pattern on a semiconductor wafer, is excellent in polishing characteristics without using slurry, and generates few scratches.
A still further object of this invention is to provide a polishing pad for semiconductor wafers, which is used as a pad in the polishing step of planarizing fine unevenness on a fine pattern on a semiconductor wafer, can have abrasive grains mixed at very high density without using slurry, and generates few scratches in spite of dispersion of abrasive grains at high density.